DSpace at VNU: A scheme of dual carrier modulation with soft-decoding for MB-OFDM MIMO systems

INTRODUCTION Multiband OFDM MB-OFDM UWB is a technique that transfers data at very high data rate [1].. By using OFDM on several frequency bands, MB-OFDM systems can support high speed d

A Scheme of Dual Carrier Modulation with

Soft-Decoding for MB-OFDM MIMO Systems

Tien Hoa Nguyen1, Nguyen Thanh Hieu1, Tran Van Tuyen1, Truong Vu Bang Giang1, and Van Duc Nguyen2

1Faculty of Electronics and Telecommunications University of Engineering and Technology

Vietnam National University, Hanoi; Email: tienhoa@vnu.edu.vn

2Institute of Electronics and Telecommunications, Hanoi University of Science and Technology

Abstract—The basic structure of the Multi-Band

Orthogo-nal Frequency Division Multiplexing (MB-OFDM) Ultra-Wide

Band (UWB) System is discussed in this paper The fixed-point

simulation platform using the WiMedia Alliance proposed by

ECMA 368 standards has been implemented An extension of

the physical layer using the Alamouti method to a

Multiple-Input Multiple-Output (MIMO) system is also presented Based

on the theoretical Dual Carrier Modulation (DCM) supported by

Media Alliance, we propose a scheme of the DCM-Modulation

and DCM-Demodulation with Soft-decoding for a MB-OFDM

MIMO system The system performance is analyzed using the

Saleh-Valenzuela channel model in terms of the bits error rate

and the transmission range for indoor environments

Keywords: UWB, MB-OFDM, BER, Saleh-Valenzuela (S-V)

channel model, STBC, fixed-point simulation, Soft-decoding

I INTRODUCTION

Multiband OFDM (MB-OFDM) UWB is a technique that

transfers data at very high data rate [1] By using OFDM

on several frequency bands, MB-OFDM systems can support

high speed data over multipath fading with severe long delay

spread In order to avoid interference to other systems,

Fed-eral Communications Commission (FCC) has given spectrum

mask for this technique In this order, the FCC allocated the

spectrum from 3.1 to 10.6 GHz for unlicensed use by UWB

transmitters operated at a limited transmission power of -41.25

dBm/MHz or less

In order to transmit at very high data speeds, two modes

of operation are required For data rate 53.3−200 Mbps the

system uses QSPK Modulation and time domain spreading

(TDS), where the first data symbol is used to form the

second symbol For data rates of 320 Mbps and higher, data

subcarriers of an OFDM symbol are modulated, using dual

carrier modulation (DCM) DCM modulates the same four

bits in two different subcarriers of the same OFDM symbol,

separated by 50 subcarriers (approximately 206 MHz), DCM

adds frequency diversity and thereby reduces the impact of

frequency-selective fading

The MB-OFDM signal is transmitted over multipath fading

channels Several channels have been proposed for high speed

UWB systems in IEEE 802.11.3a standard These channels

are CM1, CM2, CM3 and CM4 that have been built based on

Saleh Valenzuela model CM1 and CM2 cover 0 − 4 meters

with line-of-sight and non-line-of-sight CM3 and CM4 are

non-line-of-sight models CM3 is used for 4 − 10 meters and

CM4 models an extreme RMS delay spread of 25 ns

In this paper, as a solution for higher rate and reliable transmission, a MB-OFDM MIMO system is proposed A combination of space-time block codes (STBC) and hopping multiband UWB transmission is presented to exploit the spatial and frequency gain of the system The implemented system

is available for further performance evaluation with more transceiver antennas Simulation results show that the system performance with DCM provides the robustness of the UWB system to fading channel and has a gain such as frequency diversity By spreading, each OFDM symbol is sent twice over each sub-band, therefor the system also achieves a frequency diversity gain of 3dB

II MB-OFDM UWBSYSTEM DESCRIPTION

The WiMedia Alliance has already published its ECMA International Standard for UWB Systems In December 2007 the latest version ECMA 368 has been proposed The ECMA defines a standard, that divides the entire unlicensed 7.5 GHz bandwidth from 3.1 GHz to 10.6 GHz into 14 sub-bands of each 528 MHz bandwidth, with the first twelve sub-bands contains four groups and the last two sub-bands are combined

to the fifth group The data transmission is modulated with OFDM techniques in each sub-band In the conventional OFDM systems the transmitted signal is sent over one spectral allocation The ECMA 368 rules that the frequency band is changed with each recently generated OFDM symbol These frequency changing of the sub-bands depends on the Time Frequency Code TFC, that defines the frequency allocation

of the transmitter signal Figure 1 shows an example with TFC =[1, 3, 2] It means in this cace that, the first OFDM symbol is sent on band 1, the second OFDM symbol is sent

on band 3 and the information of the third OFDM symbol is sent on band 2 The transmitter signal of MB-OFDM can be illustrated mathematically as follows

s(t) = Re

(N −1

X

n=0

rn(t − nτSym)ej2πfn t

)

where rn(t) denotes the complex baseband signal of the nth

OFDM symbol, which is in the interval from 0 to τSym not equal to zero N is the number of the OFDM Symbols, τSym

is the symbol interval and fn is the center frequency of the

2011 International Conference on Advanced Technologies for Communications (ATC 2011)

4572

4224

3696

3168

0 312.5 625 937.5 1250 1562.5

BAND 2 BAND 3

BAND 1 BAND 2 BAND 3

BAND 1

Fig 1 Time-frequency representation of multiband UWB symbols with TFC

= [1, 3, 2]

nth sub-band For carrier frequency is

fn = (2904 + 528 · n)MHz with n = 1 14 (2)

The MB-OFDM system have 128 subcarriers on each OFDM

symbol (−64 ∼ −63, 0 is DC) 12 pilot carriers are used

for synchronization and located on -55, -45, -35, -25, -5, and

5, 15, 25, 35, 45, 55 Only 100 of 128 subcarriers for data

transmission are needed Ten subcarriers are guard interval and

the six remaining subcarriers are inserted with zero OFDM

signal processes a bandwidth of 528 MHz and a duration of

312.5 ns The parameters for an OFDM symbol in time domain

are represented on Table I The subcarriers are modulated with

QPSK or DCM depending on the data rate QPSK modulation

is used for data rates 200 Mbit/sec and lower DCM is used

for data rates 320 Mbit/sec and higher The current WiMedia

Alliance standard for modulation is introduced on Table II

A MB-OFDM baseband transceiver simulation model has

been implemented using fixed point Matlab/Simulink based

on the specifications of the ECMA 368 Standard The

im-plemented system includes scrambler, Reed-Solomon and

punctured convolutional coder, interleaver, QPSK or DCM

modulator, OFDM transmitter, frequency hopping and filter

On the receiver side consists of de-scrambler, Reed-Solomon

decoder, de-punctured Viterbi decoder, de-interleaver, QPSK

or DCM de-modulator, OFDM receiver, frequency de-hopping

and filter A frame-based processing is used in this simulation

model

A Interleaver und De-interleaver

The motivation of interleaving is to provide robustness

against burst errors The bit interleaving operation is

per-formed in two distinct stages Symbol interleaving, which

permutes the bits across 6 consecutive OFDM symbols,

en-ables the PHY to exploit frequency diversity within a band

group Tone interleaving, which permutes the bits across the

data subcarriers within an OFDM symbol, exploits frequency

diversity across subcarriers and provides robustness against

narrow-band interferers The symbol interleaving operation

is performed by first grouping the coded bits into blocks

of NCBP6S bits, corresponding to six OFDM symbols [1]

We denote a(i) and as(i), where i = 0, , NCBP6S − 1,

to represent the input and output bits of the symbol block

interleaver

aS(i) = a



b i

NCBPSc +

 6

NTDS



× mod(i, NCBPS)

 ,

wherebcis the floor function, mod(a, b) is the modulus opera-tor and NTDS= 2, if QPSK is used If DCM is used NTDS= 1

We denote as(j) and aT(j), where j = 0, , NCBPS− 1, represent the input and output bits of the symbol block interleaver The output of the tone interleaver is given as

aT(j) = aS



b j

NTintc + 10 × mod(j, NTint)

 , where NTint= NCBPS/NTDS The output of the tone interleaver

is then modulate with QPSK or DCM

B Proposal a mapping method for DCM Modulation After bit interleaving, the 1200 interleaved and coded bits aT(i) are the input data b[i] to QPSK or DCM mod-ulation, where i = 0, 1, 2, b[i] shall be divided into groups of 200 bits and converted into 100 complex num-bers using DCM technique The 200 coded bits are grouped into 50 groups of 4 bits Each group is represented as (bg(k), bg(k)+1, bg(k)+50, bg(k)+51), where k ∈ [0 49] and

g(k) =

( 2k k ∈ [0 24]

2k + 50 k ∈ [25 49] (3)

We denote

xg(k)= 2bg(k)− 1 (4) The two resulting DCM symbols (A[k], A[k+50]) are mapped into two 16-QAM like constellations [1],

H =



1 −2



(5)

 A[k]

A[k + 50]



= H



xg(k) jxg(k)+50

xg(k)+1 jxg(k)+51

 (6) and then allocated into two individual OFDM data subcarriers with 50 subcarriers separation Because each OFDM subcarrier occupies a bandwidth of 4.125 MHz, the bandwidth between the two individual OFDM data subcarriers is at about 200 MHz, which offers frequency diversity gain against channel deep fading

C DCM Soft bit Demapping Two equalized complex numbers, which are transmitted on different subcarriers with 50 subcarriers separation, can be combined at the receiver side to demodulate the signal The receiver signals of the kth and (k + 50)th subcarriers can be given as:

Ak= (2xg(k)+ xg(k)+1) + j(2xg(k)+50+ xg(k)+51)

Ak+50= (xg(k)− xg(k)+1) + j(xg(k)+50− xg(k)+51) (7)

we can obtain the received symbol as:

<(2Ak+ Ak+50) = 5xg(k)

=(2Ak+ Ak+50) = 5xg(k)+50

<(Ak− 2Ak+50) = 5xg(k)+1

=(Ak− 2Ak+50) = 5xg(k)+51

(8)

The decoded bit stream can be calculated with soft decoding

as follows:

bg(k)= sign(xg(k)) + 1

III MIMO MB-OFDM SCHEME

The Alamouti method has been published in several

publi-cations [2] In general, the Alamouti code is implemented,

when the space coded symbols are sent over one spectral

allocation As mentioned above, the frequency sub-band

de-pends on TFC So that there are three channels, in which the

transmitted signal is hopped and sent, (see Figure 1) To avoid

this requirement, a new process for STBC has been developed

A STBC MB-OFDM UWB transmitter

The coded data stream is first modulated using QPSK or

DCM modulator then the modulated symbols are changed

from serial to parallel The N parallel symbol vectors are given

as:

Am= [Am[0], Am[1], , Am[ND− 1]]T, (10)

where NDis the number of data-subcarriers, m = 0, 1, 2, is

the OFDM symbol index 6 OFDM Symbols [Am, , Am+5]

QPSK/DCM

Modulation

IFFT Add Pilots,

CP, GI

IFFT Add Pilots,

CP, GI

Fig 2 STBC Transmitter

are firstly grouped, then coded with the encoder matrix g2 as

follows:

gSTC2 =

-transmitted antennas

? time slots











X0 X3

X1 X4

X2 X5

−X3∗ X0∗

−X∗

4 X1∗

−X∗

5 X2∗











(11)

With the proposed coding the repeated couple signals A0, A3

and −A∗

3, A∗0 are sent in the 1st and 4th time-slots in the

same spectral allocation The designed scheme with

Mat-lab/Simulink is showed in Figure 2 Because of three

sub-bands are used to transmit Data, so that the channel matrix

H =H1 H3 H2

if TFC = [1 3 2], where H1 denotes the transfer function of the sub-band 1, H2 denotes the transfer

function of the sub-band 2, and the transfer function of the

sub-band 3 is denoted by H3

B Receiver description

During the first transmission, the symbols X0 and X3 are

transmitted simultaneously from antenna one and antenna two

respectively In the fourth transmission period, the symbol

−X∗

3 is transmitted from antenna one and the symbol X∗

0

from transmit antenna two Then we write:



Y0

Y∗

3



=



Hi1 Hi2

Hi∗

2 −Hi∗

1

 

X0

X3

 +



N0

N∗ 3

 (12)

The matrix H is defined as:

H =



Hi1 Hi2

Hi∗2 −Hi∗

1



We obtain

HHH =



Hi∗1 Hi2

Hi∗2 −Hi

1

 

Hi1 Hi2

Hi∗2 −Hi∗

1



=



|Hi

1|2+ |Hi

1|2+ |Hi

2|2



= (|Hi1|2+ |Hi2|2)I2,

(14)

where I2 is a (2 × 2) identity matrix The superscript H denotes the Hermitian (transpose conjugate) of a matrix Hi

1

and Hi

2 represent the transfer function from transmit antenna one and two to receiver antenna in the ith transmitted sub-band The receiver signal after demodulated can be written in matrix form as:

 X00

X30



=HHH

−1

HH Y0

Y3∗



=HHH

−1

HH



H X0

X3

 + N0

N3∗



= X0

X3

 + HHH
−1HH N0

N∗ 3

 (15)

IV SIMULATION RESULTS

To support the theoretical analysis given previously and implement the physical layer of a MB-OFDM system proposed

by ECMA 368, we present our simulation with employing STBC The number of antennas used in this simulation is

NTx = 2 and NRx = 1 A convolutional encoder (Rc = 5/8, 1/2) and Viterbi decoder are used for channel coding and decoding The results are represented for BER versus the transmitted bit energy to the noise power spectral density

Eb/N0 The BER for 50 channel realization results for each measurement has been averaged Figure 3 shows the results for different data rates (160, 200 and 400 Mbps), Rc = (5/8, 1/2) and different mode (SISO, MIMO) First we note that in both

Fig 3 Bit error rate Performance of UWB MB-OFDM Systems

SISO and MIMO mode the performance of system with lower

code rate Rc= 1/2 is better than higher code rate Rc= 5/8

The reason for this difference is due to redundancy At lower

code rate Rc= 1/2, the sender adds more redundant bits and

allows more efficient decoding at the receiver To compare the

data rate 200 Mbps with 400 Mbps with the same code rate

Rc and different modulation, the results show that the system

with QPSK modulation provides around 2dB in both SISO and

MIMO mode more than the DCM modulation This is due to

the fact that the complex symbols on the constellation diagram

of QPSK modulation are further to each other than DCM

modulation by the same power transmission The results show

also that the performance of the STBC MB-OFDM system

is around 5dB better than the single antenna system in all

simulated cases In the case of QPSK modulation, the STBC

achieves theoretically a diversity gain of 3dB But in low data

rate mode a significant diversity gain can be achieved by using

Time Domain Spreading (TDS) Two repeated OFDM symbols

are sent in different time slots and provides 3dB time diversity

gain So that theoretically a maximum gain of 6dB can be

achieved , and in particular the results show 5dB gain

For data rate 400 Mbps, TDS is removed in all schemes The

same symbols have been modulated to a pair of subcarriers It

means that the frequency spreading has been used instead time

domain spreading Therefore the system performance shows

also around 5dB gain when comparing MIMO using STBC

and single antenna system

∆f Subcarrier frequency spacing 4.125 MHz

TSYM Symbol duration 3.125 ns

TABLE I OFDM SYMBOL PARAMERTERS[1]

Data rate

(Mbps) Modulation Coderate FSD TSD CodedBits/Symbol

N CBPS

TABLE II ECMA 368 PARAMETERS[1]

V CONCLUSION

The physical layer and performance of MB-OFDM UWB system proposed by ECMA 368 has been investigated In this paper, we proposed a multiband MIMO coding using STBC and a scheme of mapping for coding and decoding DCM The performance of the system is evaluated with channel CM1 specified by the 802.15.3a, represents LOS with distances

of less than 4 m The developed simulation platform of MB-OFDM UWB baseband transceiver can be used directly

to change the sub-blocks and to compare the performance variations effectively for further updated specifications

VI ACKNOWLEDGMENT This work has been partly supported by the research project number QG.10.43, granted by Vietnam National University (VNU)

REFERENCES

[1] ECMA, ”Standard ECMA-368: High data rate ultra wideband PHY and MAC standard”, Dec 2005

[2] S M Alamouti, ”A simple transmit diversity technique for wireless communication”, IEEE Journal on Select Areas in Com-munications, Vol 17, pages(s) 1451-1458, Oct.1998

[3] IEEE P802.15.3a, ”IEEE 802.15.3a high datarate alternative PHY Task Group (TG3a) for wireless personal area networks: channel modeling subcommitee report (Doc Number P802.15-02/368, SG3a”, Sep 2002

[4] W Pam Siriwongpairat and K J Ray Liu, ”Ultra-Wideband Communications Systems: Multiband OFDM Approach”, Wiley IEEE Press, December 2007

[5] Ghobad Haidari ”WiMidia UWB Technology of Choice for Wireless UWB and Bluetooth”, Wiley, September 2008 [6] Mohammad Ghavami, Lachlan Michael, and Ryuji Kohno, ”Ultra Wideband Signals and Systems in Communication Engineering”, John Wiley and Sons, May 2004

[7] A Batra et al, ”Multi-band OFDM physical layer proposal for ieee 802.15 task group 3a”, IEEE P802.15.3a, page 268r3, July